An actuator is a device that performs some mechanical action. One actuator is a piston, wherein a plunger of the piston moves in a reciprocating manner. The plunger can therefore be connected to some manner of work piece or other mechanical system.
While some actuators are used to move a work piece, other actuators are employed to maintain a constant position of a work piece. This can become difficult when the force applied by or on the work piece is not constant. Under variable forces, the actuator may tend to oscillate between optimal positions.
One attempt to address this problem has been to provide a pneumatic actuator. A pneumatic actuator provides pneumatic pressure to one side of a plunger. The pressure provided can be supplied such that the plunger either remains in a fixed position or is moved in a desired direction. The problem with an actuator employed to maintain a desired position is that when the force on the actuator changes, the pneumatic pressure supplied to the actuator must also change in order to maintain the desired position. For example, if the force applied to the actuator increases, the pneumatic pressure must also increase. Similarly, if the force applied to the actuator decreases, the pneumatic pressure must also decrease.
A problem in the past has been the ability to adjust the pneumatic pressure in response to changes in the force applied on the actuator. One method employed incorporates positioning a shift valve on the work piece and having the shift valve communicate pneumatically with the actuator. The shift valve moves with the actuator and so if the actuator moves, so does the shift valve. Once the actuator moves a pre-determined distance, the shift valve either opens or closes to allow the pneumatic pressure in the actuator to increase, decrease, or remain constant. The problem with incorporating a shift valve is that a shift valve typically has a constant cross sectional area for air to flow and therefore, only knows the positions “open” and “closed.” Therefore, the cross-sectional area of these shift valves must not be chosen too big, because a quick reaction on short strokes would cause resonance oscillations. However, a quick reaction time is typically desired. Therefore, the cross-sectional area of the valve must be balanced against the risk of resonance oscillations. Because of this compromise, shift valves typically have the inherent problem of overshooting the target position. This is because the same amount of air is provided/released regardless of how close the actuator is to its desired position.
Therefore, there is a need in the art to provide a proportional actuator valve which adequately maintains the position of an actuator and can adjust the position of the actuator.